Traditional mill control and operation has been acceptable for the rolling of single billets, but with the advent of the new technology of continuously welded billets, control strategies and operations have to be rethought to obtain the full benefits of the process. These benefits include increased yield and productivity, reduced cobbles and more consistent tolerances. However, the welding process raises the joint temperature above the remainder of the billet and produces a small region that is softer and less resistant to rolling forces. This high temperature can produce excessive dimensional change at the welded joint outside of the specified tolerance for the product and is unacceptable.
The behavior of the rolled material through a roll pass in hot rolling is governed by many factors. The most variable of these factors is the temperature of the material. Higher temperature material has a tendency to elongate more during rolling while colder material will spread more. The change in elongation and spread results in a variation in product exit speed.
In continuous rolling mills, the material can be present in many pairs of rolls at the same time and the relative speeds of the roll pairs must be balanced to avoid either accumulation of material between stands of the roll pairs or tension in the material.]
In so-called roughing stands, automatic tension control is set solely on the conditions at the head of the billet. Any deviation of temperature between the head of the billet and the remainder of the billet will result in an incorrect speed setting. A cold head can result in an accumulation of material between the stands and a hot head in tension in the material.
The accumulation of material between stands is an unstable and hazardous condition and operators normally set the relative speeds to avoid this condition and thus carry out rolling with tension in the roughing stands. It is this tension that is the cause of the dimensional change in the welded joint.
Control can be continuously applied for material of small cross-section by the use of loopers in the intermediate and finishing stands. Loopers control the relative roll speeds in the stands by measuring the displacement of a loop of material formed between adjacent stands. Arising loop increases the speed differential, and a falling loop decreases the differential by adjusting the upstream stand. The transient effect of the weld joint is to briefly raise the loop height but only after the weld has passed the stand. The speed control is thus applied to the wrong part of the material.